Magnetism and structural phase transitions in LiTmF4 powders

The field (0–5.5 T) and temperature (2–300 K) dependences of the magnetization of LiTmF₄ powders with particle sizes of 1 μm and 56–400 μm are investigated experimentally and theoretically. It is concluded that a transition layer exists between the thulium ions in the bulk and the ions at the surface. Two magnetic-field-induced structural phase transitions are observed at low temperatures, and the temperature dependence of the critical magnetic fields is established. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 4, 247–250 (25 August 1997)     

Diffraction of picosecond bulk longitudinal and shear waves in micron thick films

Investigation of thin metallic film properties by means of picosecond ultrasonics [C. Thomsen et al., Phys. Rev. Lett. 53, 989 (1984)] has been under the scope of several studies. Generation of longitudinal and shear waves [T. Pézeril et al., Phys. Rev. B 73, 132301 (2006); O. Matsuda et al., Phys. Rev. Lett. 93, 095501 (2004)] with a wave vector normal to the film free surface has been demonstrated. Such measurements cannot provide complete information about properties of anisotropic films. Extreme focusing of a laser pump beam (≈0.5 μm) on the sample surface has recently allowed us to provide evidence of picosecond acoustic diffraction in thin metallic films (≈1 μm) [C. Rossignol et al., Phys. Rev. Lett. 94, 166106 (2005)]. The resulting longitudinal and shear wavefronts propagate at group velocity through the bulk of the film. To interpret the received signals, source directivity diagrams are calculated taking into account material anisotropy, optical penetration, and laser beam width on the sample surface. It is shown that acoustic diffraction increases with optical penetration, so competing with the increasing of directivity caused by beam width. Reflection with mode conversion at the film-substrate interface is discussed.     

Giant enhancement of sum-frequency generation upon excitation of a surface plasmon-polariton

The generation of the sum frequency of visible (0.5235 μm) and IR (10 μm) radiation on smooth and corrugated silver surfaces is investigated. The sum-frequency signal obtained with a visible-range surface plasmon-polariton excited on a corrugated silver-air interface is found to be more than four orders of magnitude stronger than the signal from a smooth silver surface. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 9, 581–584 (10 November 1997)     

Fractal structure of fullerite

We have obtained macroscopic fractal aggregates of fullerite ranging in size from 100 to 400 μm. The fractal structure of fullerite is formed by microscopic (30–100 nm) fullerite crystals in the presence of a temperature gradient under conditions of diffusion-limited aggregation. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 9, 684–685 (10 May 1998)     

Generation of mesoscopic magnetic structures by local laser action

New effects are observed wherein the internal structure of the domain walls in a thin magnetic iron garnet film are modified by the action of focused laser radiation. A single laser pulse with increasing power gives rise to the following: 1) displacement of vertical Bloch lines in a domain wall; 2) generation of a pair of vertical Bloch lines on initially line-free walls; and, 3) an irreversible change in shape of a domain wall and the domain structure as a whole. The mechanism leading to the generation and displacement of Bloch lines is connected with the motion of domain walls which is induced by a local change in the distribution of demagnetizing fields as a result of a heating-induced decrease of the magnetization in the focal spot of the laser radiation. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 6, 398–402 (25 September 1997)     

Structure and phase composition of a chromium-silicon system modified by high-current electron beams

The results of studies of the structure-phase state of a chromium-coated silicon substrate system’s subsurface layer treated with low-energy high-current electron beams, 50–200 μs in duration and with an energy density of 15 J/cm², are reported. The data of raster electron microscopy and X-ray structural and spectral microanalysis revealed the formation of a chromium-doped silicon layer with a thickness of 2–38 μm, chromium-enriched silicon dendrites, chromium disilicide CrSi₂, and an amorphous eutectic layer (the characteristic cross-section size of the chromium-enriched phase extrusions is ∼50 nm). The structure-phase transformations are discussed taking into account the peculiarities of the distribution of temperature, diffusion and convective mass-transfer in the modified layer.     

Spontaneous far-IR emission accompanying transitions of charge carriers between levels of quantum dots

The spontaneous emission of far-infrared radiation (λ≅10–20 μm) from diode structures with vertically coupled InGaAs/AlGaAs quantum dots is observed. This emission is due both to transitions of holes and electrons between size-quantization levels in quantum dots and to transitions from the continuum to a level in a quantum dot. It is observed only when accompanied by lasing at short wavelengths (λ≅0.94 μm) and, like the short-wavelength emission, it exhibits a current threshold. The spontaneous emission of long-wavelength radiation is also observed in InGaAs/GaAs quantum-well laser structures. This radiation is approximately an order of magnitude weaker than that from quantum-dot structures, and it has no current threshold. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 4, 256–260 (25 February 1998)     

Anomalous burn-through of thin foils by high-intensity laser radiation

The paper presents results of experiments performed on the Pico facility in which foils were heated by laser radiation, and anomalously fast burn-through of foils by a structured laser beam was detected. Comparison with two-dimensional calculations has allowed us to suggest a tentative mechanism for the effect under investigation. The targets in the experiments were thin aluminum foils of thickness 3 to 40 μm. The flux density of laser radiation on the target surface varied between 10¹³ and 10¹⁴ W/cm². We detected a strong dependence of the transmitted energy on the foil thickness and the shortening of the transmitted laser pulse. Penetration of laser radiation through foils with thicknesses considerably larger than 3 μm has been observed, although it was stated in earlier publications [V. V. Ivanov, A. K. Knyazev, A. V. Kutsenko, et al., Kratk. Soobshch. Fiz. FIAN No. 7–8, 37 (1997)]; A. é. Bugrov, I. N. Burdonskii, V. V. Gol’tsov et al., Zh. éksp. Teor. Fiz. 111, 903 (1997) [JETP 84, 903 (1997)] that, at the laser radiation parameters used in our experiment, the evaporated layer of the foil could not be thicker than 2 μm. Two-dimensional calculations have allowed us to interpret this effect in terms of local “piercing” of the target at spots on the target surface where the radiation intensity has its peaks. The possibility of reducing these peaks by using a symmetrizing prepulse is discussed in the paper. Zh. éksp. Teor. Fiz. 116, 1287–1299 (October 1999)     

Ordered structures in a nonideal dusty glow-discharge plasma

The formation of ordered structures of charged macroparticles in a constant-current neon glow-discharge plasma is investigated. Experiments were performed with two types of particles: thin-walled glass spheres 50–63 μm in diameter and particles of Al₂O₃, 3–5 μm in diameter. Formation of quasicrystalline structures is observed in the standing strata and in an artificially created double electric layer. The formation of extended filamentary structures of macroparticles in the absence of visible stratification of the positive column has been observed for the first time. The influence of the discharge parameters on the formation of the ordered structures and their melting is examined. The form of the interaction potential between the charged macroparticles is considered, as well as changes in the conditions for maintaining the discharge in the presence of high concentrations of dust particles. Zh. éksp. Teor. Fiz. 112, 2030–2044 (December 1997)     

Dynamics of domain walls in ultrathin magnetic films

The domain walls in ultrathin ferromagnetic films with uniaxial magnetic anisotropy are investigated theoretically. It is shown that taking account of the magnetodipole and magnetoelastic interactions leads to the appearance of an effective anisotropy with respect to the direction of the normal to the plane of the wall. The existence of a new type of domain walls—“corner” walls, at which the magnetization vector is rotated in the plane making a certain angle, which depends on the film parameters, with the plane of the domain wall and the static and dynamic properties of these walls are investigated. The dependence of the limiting velocity of the domain walls on the film thickness is found. Zh. éksp. Teor. Fiz. 112, 1476–1489 (October 1997)     

Crystallization of a dusty plasma in the positive column of a glow discharge

The formation of macroscopic ordered structures in the standing striations of a stationary glow discharge in Ne is observed. A Coulomb quasicrystal is formed by spherical glass particles with diameters of 50–63 μm and charge Z _p~7·10⁵ e. The interparticle distance is approximately 300 μm. This corresponds to a nonideality parameter Γ~5·10⁴, which leads to crystallization in the Yukawa model. The factors leading to the formation of a quasicrystal in the striations are discussed. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 2, 86–91 (25 July 1996)     

Local surface segregations of implanted aluminum in an iron crystal with a low density of defects

The surface distribution of elements is studied by scanning a 3-MeV proton beam along the surface of a bcc-Fe sample implanted with aluminum ions in the dose interval (1–50) · 10¹⁶ cm⁻². Ring-shaped regions, up to 30 μm in diameter, with a high density of aluminum, which appear at implantation doses (5–20) · 10¹⁶ cm⁻², are observed. These regions appear as a result of radiation-stimulated segregation processes. A mechanism based on the existence of a low density of dislocations in the initial crystal is proposed to explain the implanted impurity segregation processes. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 1, 86–89 (10 January 1997)     

Achromatic reconstruction of the wave front of femtosecond laser pulses

Wave-front reconstruction by geometric-optical reflection of the reconstructing radiation from interference surfaces of a structure recorded in the bulk of a medium by counterpropagating laser pulses is observed. The recording is done with the aid of a sapphire titanate laser. Pulse durations of 30–40 fs are used. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 5, 388–390 (10 March 1997)     

Experimental investigation of the characteristic features of bremsstrahlung from high-energy electrons in thin amorphous targets

Radiation from 0–900 MeV electrons in thin amorphous films is investigated experimentally in the photon energy range 20–700 keV. The Landau-Pomeranchuk-Migdal suppression of the soft part of the bremsstrahlung spectrum and the Ter-Mikaélyan density effect are detected. Coherent bremsstrahlung on macroscopic inhomogeneities in the target material is observed. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 5, 369–373 (10 March 1997)     

Characteristic features of the x-ray spectra of a plasma produced by heating CO2 clusters by intense femtosecond laser pulses with λ=0.8 and 0.4 μm

The x-ray spectra of a plasma produced by heating CO₂ clusters with intense femtosecond laser pulses with λ=0.8 μm and λ=0.4 μm are investigated. Spatially resolved x-ray spectra of the cluster plasma are obtained. The observed characteristic features of the x-ray emission spectra show unequivocally that such a plasma contains quite a large relative number of ions (≃10⁻²–10⁻³) with energies of 0.1–1 MeV. The contour of the OVIII Ly_α line is found to have characteristic features that are especially conspicuous when the clusters are heated with second-harmonic pulses. These features cannot be explained by any mechanisms known to the authors. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 6, 454–459 (25 September 1998)     

“Unusual” domain walls in multilayer systems: Ferromagnet + layered antiferromagnet

The structure and conditions for the onset of a new type of domain wall in multilayer systems comprising a ferromagnet and a layered antiferromagnet is investigated by numerical simulation. Domain walls occur as the result of frustrations produced by interface roughness, i.e., by the existence of atomic steps on them. The domain walls are investigated both in a ferromagnetic film on a layered antiferromagnetic substrate and in multilayer structures. It is shown that a domain wall broadens with increasing distance from the interface; this trend is attributed to the nontrivial dependence of the wall energy on the thickness of the layer. The structure of the domain walls in multilayer ferromagnet-layered antiferromagnet systems varies dramatically as a function of the energies of interlayer and in-layer exchange interactions between adjacent layers. Zh. éksp. Teor. Fiz. 114, 1817–1826 (November 1998)     

Phase composition and defect substructure of nickel alloyed with boron and copper by electric explosion of conductors

Using methods of electron microscopy, we have discovered that combined electroexplosive alloying of nickel with boron and copper gives rise to a multilayer structure. An outer amorphous crystalline layer incorporating 2–3 nm nickel-and copper-boride, oxide, and boride-oxide crystallites is formed on the alloyed surface. The intermediate (subsurface) layer 1–2 μm thick is made up of NiB ₁₂ and Ni ₄ B ₃ crystallites 120–130 nm in size, with boron-and copper-oxide particles observed along the grain boundaries. The underlying thick layer exhibits a cellular Ni-Cu-B melt crystallization structure grading initially into a high-rate dendritic crystallization structure and then into a granular structure. The electroexplosive alloying process is found to bring into existence a high scalar dislocation-density substructure in nickel crystallites both in the alloyed zone and in the adjacent heat-affected zone. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 3–7, March 2007.     

Nonlinear laser-induced regime of surface-electromagnetic-wave generation and submicron periodic relief during liquid-phase photochemical etching of n-III–V semiconductors

This paper is devoted to an experimental study of the physical processes underlying the phenomenon of laser-induced generation of periodic relief on the surface of n-III–V semiconductors during liquid-phase photochemical or photoelectrochemical etching accompanying the resonance interaction of surface electromagnetic waves (SEWs). The increments of the exponentially increasing amplitudes of the dominant Fourier harmonics of the relief have been measured at the initial (linear) stage of the time evolution of the surface profile. It is proven by comparing the theoretical and experimental results that the mechanism for forming periodic structures that we have proposed is adequate. Ways of monochromatizing the generated relief and controlling the line shape of the surface grating are studied. It is experimentally detected for the first time that the nonlinear stage of the time evolution of the relief is characterized, in accordance with the predictions of the theory developed by the authors, by amplitude and phase oscillations of the first and second Fourier harmonics of the surface profile. It is shown to be possible to generate relief that suppresses specular reflection from the surface. A new nonmasked laser method is developed for forming high-quality submicron relief diffraction gratings, combining a holographic method and a method involving laser-induced relief generation during resonance excitation of SEWs. Diffraction gratings with a period of d=0.24–0.54 μm and a depth of h=0.1–0.2 μm over an area of 0.5×31 cm have been created on an n-InP surface. Zh. éksp. Teor. Fiz. 111, 174–198 (January 1997)     

Current hysteresis collapse and the formation condition for electric-field domains in lightly doped superlattices

It is shown that under resonance tunneling conditions the energy structure of electric-field domains in doped superlattices is substantially different from a resonance structure. As the impurity concentration decreases, the domain walls become wider and the current hysteresis becomes narrower (collapses) as a result of the mismatch of the resonance levels in the region of a domain wall. A physical interpretation of the criterion for the existence of electric-field domains is given. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 3, 146–151 (10 August 1996)     

Suppression of the fractal conductivity channel and superlocalization effects in porous a-Si:H

The temperature dependence of the hopping conductivity and the relaxation kinetics of the transient current in porous amorphous silicon are investigated after treatment in a hydrogen plasma at 200 °C. It is discovered that posthydrogenation of the material increases the dimension of the conducting channel from 2.5 to 3, while suppressing and slowing the relaxation of the transient current. The results obtained are attributed to passivation of the electrically active dangling bonds on the pore surface by hydrogen. It is concluded that electron transport in porous amorphous silicon in the temperature range T>T*, where T* lies in the range 130–270 K and depends on the density of states, takes place between superlocalized states of the internal surface, which is enriched with dangling bonds and acts as a fractal percolation system. When the temperature is lowered below T*, a transition to one-dimensional hopping conduction in the bulk silicon regions occurs. Zh. éksp. Teor. Fiz. 112, 926–935 (September 1997)